1. Field of the Invention
The present invention generally relates to voltage controlled amplifiers (VCAs), and more specifically to a current mode VCA that provides a constant wideband frequency response over a large gain range by maintaining constant loop gain and linearizing the loop gain for large input signals.
2. Description of the Related Art
VCAs are used to provide gain as an exponential function of a control parameter and to amplify an input signal by the selected amount of gain. In general, as the amount of gain extracted from the VCA is increased its loop gain is reduced, which in turn reduces the VCA's cut-off frequency so that its gain-bandwidth product remains constant. The result is that the effective bandwidth of the VCA is reduced at higher gain values.
VCAs are commonly used in audio systems to control the compression of an audio signal. A control circuit adjusts the VCA's control parameter as a function of the audio signal, typically its root-mean-square (RMS) value, so that the gain of the VCA is controlled to prevent clipping of audio signals that are too large and noise amplification of audio signals that are too small. This improves the perceptual quality or vocal clarity of the audio signal.
The VCA described in a product data sheet for a "Voltage-Controlled Amplifier/OVCE," model SSM-2018 produced by Analog Devices, Inc., 1981, uses a negative feedback architecture around a gain core stage to produce a differential output current and a differential feedback current. A transresistance stage comprising a pair of resistors responds to a differential input voltage and the differential feedback current to produce a differential voltage. A one pole transconductance stage uses a capacitor to limit the VCA's high frequency gain to stabilize the feedback loop and supply a differential drive current to the gain core stage. The VCA also includes a second gain core stage and a differential pair of transistors for adjusting the effective capacitance of the transconductance stage to compensate for changes in the amount of gain extracted from the VCA. This maintains a constant loop gain, and hence a constant frequency response.
The second gain core stage and its control circuitry use a large number of components to maintain the loop gain, which increases cost and reduces accuracy, and only maintains the constant frequency response for a 20 dB change in gain. The VCA's voltage feedthrough makes it very sensitive to transistor mismatch, and requires two trims to zero the resulting offset voltage. Furthermore, the VCA would preferably have better linearization of its loop gain which would increase its bandwidth.